Page 545 - Basic Electrical Engineering

The no-load power input is wasted as a loss as there is no output.

Input = output + losses

If output is zero, input equals losses. Let us see what are the no-load losses.

Since current I is flowing through the primary winding which has a
0
resistance of, say R , there will be some amount of copper loss in the winding
1
as R Watts. However, since I is small, R will also be small. Since the
1
1
0
core is made up of a magnetic material there will be loss in the core. The core
loss is due to two reasons. One is called hysteresis loss. Hysteresis loss is
caused due to the magnetization of the magnetic material in alternate
directions in every half cycle of the supply voltage. The magnetic dipoles of

the magnetic core material align themselves in alternate directions producing
alternating flux. The work done due to this is equivalent to the input energy

spent and is called hysteresis loss.
The other loss component is due to eddy current. Large number of small

eddy currents flow in the magnetic core material due to the EMF induced in
the core, which is subjected to alternating magnetic field similar to the two

windings. EMFs get induced in the core material for the same reason as for
the coils. This EMF induced in the core creates current which continues to
circulate in the core and heat up the core unnecessarily. This current is of no

use to us and leads to only waste of input energy. This is called eddy current
loss. The sum of hysteresis loss (W ) and eddy current loss (W ) is called
h
e
core loss (W ) or iron loss (W ). The no-load input power is expressed as
c
i

If we neglect the small amount of R then, no-load input power
1

W = W + W = W , i.e., equal to core loss
e
h
0
c
6.7 TRANSFORMER ON LOAD